# std::copy, std::copy_if

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt, class OutputIt > OutputIt copy( InputIt first, InputIt last, OutputIt d_first ); | (1) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2 > ForwardIt2 copy( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first ); | (2) | (since C++17)
template< class InputIt, class OutputIt, class UnaryPred > OutputIt copy_if( InputIt first, InputIt last, OutputIt d_first, UnaryPred pred ); | (3) | (since C++11) (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class UnaryPred > ForwardIt2 copy_if( ExecutionPolicy&& policy, ForwardIt1 first, ForwardIt1 last, ForwardIt2 d_first, UnaryPred pred ); | (4) | (since C++17)

Copies the elements in the range, defined by `[`first`,` last`)`, to another range beginning at d_first (copy destination range).

1) Copies all elements in the range `[`first`,` last`)` starting from first and proceeding to last.
If d_first is in `[`first`,` last`)`, the behavior is undefined. In this case, std::copy_backward may be used instead.
2) Copies the elements, but executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

If `[`first`,` last`)` and the copy destination range overlaps, the behavior is undefined.
3) Only copies the elements for which the predicate pred returns true. This copy algorithm is stable: the relative order of the elements that are copied is preserved.
If `[`first`,` last`)` and the copy destination range overlaps, the behavior is undefined.
4) Same as (3), but executed according to policy.
This overload participates in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. | (since C++20)

### Parameters

first, last | - | the range of elements to copy
d_first | - | the beginning of the destination range
policy | - | the execution policy to use
pred | - | unary predicate which returns ​true for the required elements. The expression pred(v) must be convertible to bool for every argument `v` of type (possibly const) `VT`, where `VT` is the value type of `InputIt`, regardless of value category, and must not modify `v`. Thus, a parameter type of VT&is not allowed, nor is VT unless for `VT` a move is equivalent to a copy(since C++11). ​
Type requirements
- `InputIt` must meet the requirements of LegacyInputIterator.
- `OutputIt` must meet the requirements of LegacyOutputIterator.
- `ForwardIt1, ForwardIt2` must meet the requirements of LegacyForwardIterator.
- `UnaryPred` must meet the requirements of Predicate.

### Return value

Output iterator to the element in the destination range, one past the last element copied.

### Complexity

Given \(\scriptsize N\)N as std::distance(first, last):

1,2) Exactly \(\scriptsize N\)N assignments.
3,4) Exactly \(\scriptsize N\)N applications of the predicate pred, and at most \(\scriptsize N\)N assignments.

For the overloads with an `ExecutionPolicy`, there may be a performance cost if `ForwardIt1`'s value type is not MoveConstructible.

### Exceptions

The overloads with a template parameter named `ExecutionPolicy` report errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

copy (1)
    template<class InputIt, class OutputIt>
    OutputIt copy(InputIt first, InputIt last,
                  OutputIt d_first)
    {
        for (; first != last; (void)++first, (void)++d_first)
            *d_first = *first;

        return d_first;
    }
copy_if (3)
    template<class InputIt, class OutputIt, class UnaryPred>
    OutputIt copy_if(InputIt first, InputIt last,
                     OutputIt d_first, UnaryPred pred)
    {
        for (; first != last; (void)++first)
            if (pred(*first))
            {
                *d_first = *first;
                (void)++d_first;
            }

        return d_first;
    }

### Notes

In practice, implementations of `std::copy` avoid multiple assignments and use bulk copy functions such as std::memmove if the value type is TriviallyCopyable and the iterator types satisfy LegacyContiguousIterator.

When copying overlapping ranges, `std::copy` is appropriate when copying to the left (beginning of the destination range is outside the source range) while `std::copy_backward` is appropriate when copying to the right (end of the destination range is outside the source range).

### Example

The following code uses `std::copy` to both copy the contents of one std::vector to another and to display the resulting std::vector.

Run this code

    #include <algorithm>
    #include <iostream>
    #include <iterator>
    #include <numeric>
    #include <vector>

    int main()
    {
        std::vector<int> from_vector(10);
        std::iota(from_vector.begin(), from_vector.end(), 0);

        std::vector<int> to_vector;
        std::copy(from_vector.begin(), from_vector.end(),
                  std::back_inserter(to_vector));
    // or, alternatively,
    //  std::vector<int> to_vector(from_vector.size());
    //  std::copy(from_vector.begin(), from_vector.end(), to_vector.begin());
    // either way is equivalent to
    //  std::vector<int> to_vector = from_vector;

        std::cout << "to_vector contains: ";

        std::copy(to_vector.begin(), to_vector.end(),
                  std::ostream_iterator<int>(std::cout, " "));
        std::cout << '\n';

        std::cout << "odd numbers in to_vector are: ";

        std::copy_if(to_vector.begin(), to_vector.end(),
                     std::ostream_iterator<int>(std::cout, " "),
                     [](int x) { return x % 2 != 0; });
        std::cout << '\n';

        std::cout << "to_vector contains these multiples of 3: ";

        to_vector.clear();
        std::copy_if(from_vector.begin(), from_vector.end(),
                     std::back_inserter(to_vector),
                     [](int x) { return x % 3 == 0; });

        for (const int x : to_vector)
            std::cout << x << ' ';
        std::cout << '\n';
    }

Possible output:

    to_vector contains: 0 1 2 3 4 5 6 7 8 9
    odd numbers in to_vector are: 1 3 5 7 9
    to_vector contains these multiples of 3: 0 3 6 9

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 2039 | C++11 | the return value of `std::copy_if` was not specified | specified
LWG 2044 | C++11 | the stability of `std::copy_if` was not defined | defined

### See also

copy_backward | copies a range of elements in backwards order (function template)
reverse_copy | creates a copy of a range that is reversed (function template)
copy_n (C++11) | copies a number of elements to a new location (function template)
fill | copy-assigns the given value to every element in a range (function template)
remove_copyremove_copy_if | copies a range of elements omitting those that satisfy specific criteria (function template)
ranges::copyranges::copy_if (C++20)(C++20) | copies a range of elements to a new location (algorithm function object)

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